The 51 states (including the District of Columbia) that provided enrollment data for December 2014 reported nearly 69.7 million individuals were enrolled in Medicaid and CHIP. This enrollment count is point-in-time (on the last day of the month) and includes all enrollees in the Medicaid and CHIP programs who are receiving a comprehensive benefit package.

547,263 additional people were enrolled in December 2014 as compared to November 2014 in the 51 states that reported comparable December and November data.

(And yes, the "51 states" wording is CMS's, not mine)

Looking at the additional enrollment since October 2013 when the initial Marketplace open enrollment period began, among the 49 states reporting both December 2014 enrollment data and data from July-September of 2013, over 10.75 million additional individuals are enrolled in Medicaid and CHIP as of December 2014, approximately an 18.6 percent increase over the average monthly enrollment for July through September of 2013. (Connecticut and Maine are not included in this count.)

Among states that had implemented the Medicaid expansion and were covering newly eligible adults in December 2014, Medicaid and CHIP enrollment rose by over 27 percent compared to the July-September 2013 baseline period, while states that have not, to date, expanded Medicaid reported an increase of over 7 percent over the same period.

Fifteen of the 26 states that had implemented the Medicaid expansion and were covering newly eligible adults in December 2014, and that reported relevant data for both December and the July-September 2013 baseline period, experienced an enrollment increase of 30 percent or more. 11

These enrollment counts are in addition to the enrollment increases from the nearly 950,000 individuals who gained coverage as a result of the Affordable Care Act before open enrollment began. Seven states implemented an “early option” to expand Medicaid coverage to adults with incomes up to 133 percent of the FPL between April 1, 2010 and January 1, 2014, using new state plan authority provided by the Affordable Care Act or a demonstration under section 1115 of the Social Security Act building upon that authority.

If you add those 950K in, the total net increase in Medicaid/CHIP enrollment due to the Affordable Care Act is closer to 11.7 million through the end of November.

Of course, that was almost months ago. Since then, we've turned over to a new calendar year; this number doesn't include anyone who enrolled (or re-enrolled?) for January 2015. I'm not sure how Medicaid renewals ("redeterminations") work; presumably 2014 enrollees had to re-apply for 2015 just like private QHP enrollees do, so it's likely that the shift from December to January on my graph should actually show an abrupt, dramatic drop right at the end of December, followed by a steep climb back up. Unfortunately, due to the nature of Medicaid reporting, there's no way of being sure about this. All I have to go on for the moment is the ASPE report through 1/16, which includes all exchange-based Medicaid enrollments...

2,326,124 Medicaid/CHIP enrollees via the state-based exchanges

904,678 Medicaid/CHIP enrollees via HC.gov

...or just 3.23 million. Unfortunately, as you can tell from the skewed numbers above, this isn't terribly useful, since most Medicaid enrollments (whether "traditional" or via ACA expansion) aren't run through the ACA exchanges, but through traditional means (the state social services office, etc). A couple of states have started running all Medicaid/CHIP enrollment through their exchanges, but that's few and far between, so this number doesn't mean a whole lot, I'm afraid.

Anyway, with the addition of Pennsylvania and Indiana to the ACA expansion list (even if both states are doing it in a confusing-as-hell, complicated manner), the numbers should continue to rise throughout the year...

Naturally, the states which are still refusing to expand Medicaid are dragging things down:

Among states that had implemented the Medicaid expansion and were covering newly eligible adults in December 2014, Medicaid and CHIP enrollment rose by over 27 percent compared to the July-September 2013 baseline period, while states that have not, to date, expanded Medicaid reported an increase of over 7 percent over the same period.